With the Third Years away at Camelot College, Cackle's is in the care of Miss Drill and Miss Crotchet. The girls return from the Inter-Schools Festival of Music and Chanting having won First Prize. Professor von Raffenburg, a world-famous conductor who was at the festival, comes to Cackle's as he is impressed with Miss Crotchet's musical compositions.
He invites the choir to give a performance in Vienna. Clarice doesn't hear properly because she has cotton in her ears (she's sick). Crotchet invites him to stay over and hear the choir practice. However, Sybil starts to doubt the Professor's motives when he appears unable to play any instrument or know anything about music.
Von Raffenburg later tells Miss Crotchet of his ambition to find the 'Lost Chord' - a legendary chord which is said to have the power to grant wealth and happiness to anyone who hears it. The instruction of how to hear the Chord is written in an ancient book called 'Magicus Musicalis', but there is only one copy in existence. Miss Crotchet discovers the book is in the school library, but it contains forbidden spells and decides not to tell the Professor.
Unbeknownst to Miss Crotchet, the Professor finds the book and persuades Fenella and Griselda to help him make a magic bell to hear the Lost Chord. They scrounge up ingredients and meet in the entrance hall that night. He needs silver and he wants to melt their award. They don't like the idea, but he talks them into it. They make a bell. He puts it in a box and says goodbye until morning, when he will present it to Miss Crotchet.Meanwhile, Sybil has worked out that the Professor is a conman and asks Clarice to help her tell Miss Crotchet, but Clarice is ill in bed with cotton wool in her ears.
During assembly, the Professor presents the bell to Miss Crotchet. When she rings it, the Lost Chord takes effect, freezing everyone who hears it with a continuous sound. (Von raffenburg is wearing earmuffs, so it doesn't effect him). The Professor then sets about robbing the castle of its treasures. Clarice, not being able to hear the Chord, discovers the Professor robbing the girls' rooms.
She goes down to the great hall and waves her hand in front of people's faces. They're all still frozen. She goes over to Sybil whose eyes are still moving. Communicating only with Sybil's eyes, Clarice figures out, "You can hear something I can't?" She plugs Sybil's ears and Sybil snaps out of it. Sybil plugs her own ears and frees Griselda. They give her the cotton and tell her to free everyone else.
The Professor's just about to go out the main gate when Sybil magics it closed. Everyone comes running out and he admits that he's a fraud. He asks Miss Crotchet to come with him anyway, "with your powers and my skill we can have everything!" She casts a spell and his cymbals follow him, crashing nastily as he runs away into the woods. Miss Crotchet turns the bell back into the trophy.
The Worst Witch TV Series